1. Field of the Invention
The present invention relates to circuit board transferring apparatus and method and a solder ball mounting method, for example, an apparatus for arranging a circuit board of a plurality of BGA semiconductor packages before solder balls are mounted on a tray and transferring it and a method for mounting solder balls on the circuit board of the BGA semiconductor packages transferred on the tray.
2. Description of the Related Art
Because of requests for advancement in functions and decrease in size and weight of an electronic unit in recent years, an internal circuit of a semiconductor package used for an electronic unit becomes complex and the number of signals to be transferred to and from an external unit is increased. Thereby, decrease in size and increase in the number of pins of a semiconductor package are progressed. As a high-density mounting technique corresponding to decrease in size and increase in the number of pins of a semiconductor package, a technique referred to as BGA (Ball Grid Array) or CSP (Chip Size Package) is used these days.
The BGA is a surface mounting component using solder balls as pins, which makes it possible to decrease a pin pitch compared to a conventional inserting component which is soldered by inserting a lead frame into a hole opened in a substrate. The CSP is a very small LSI making it possible to make the size of a semiconductor package almost equal to the size of a semiconductor chip in the package by further decreasing an pin pitch, which is one type of the BGA.
A plurality of pads are formed like a grid array for mounting solder balls on the outside of a BGA board. To mount solder balls on the BGA board, an adhesive flux is applied onto the pads, the solder balls are transferred from a ball housing vessel onto the pads to temporarily fix the solder balls on the pads by the adhesion of the flux. Thereafter, the solder balls are welded to the pads by adding heat treatment (reflow).
Solder ball transferring apparatuses have been provided so far which respectively make it possible to transfer a plurality of solder balls onto pads at the same time (for example, refer to Patent Documents 1 to 4). [Patent Document 1] Japanese Patent Laid-Open No. 8-236916; [Patent Document 2] Japanese Patent Laid-Open No. 10-189666; [Patent Document 3] Japanese Patent Laid-Open No. 11-138257; [Patent Document 4] Japanese Patent Laid-Open No. 2001-110933.
This type of the solder ball transferring apparatuses is constituted by including a ball housing vessel for housing solder balls and a ball carrier for transferring the solder balls onto pads of a BGA board from the ball housing vessel. The ball carrier has a plurality of solder ball attracting nozzles arranged on the same array as the pad array on the BGA board. The solder ball transferring apparatus thus constituted operates as described below.
Firstly, the apparatus vacuum-attracts and picks up a plurality of solder balls housed in the ball housing vessel by using solder ball attracting nozzles of the ball carrier. In this case, one nozzle vacuum-attracts one solder ball. Therefore, by carrying attracted and held solder balls onto pads and canceling the vacuum-attraction state, a plurality of solder balls are transferred onto a plurality of pads at the same time.
Thus, when transferring a plurality of solder balls onto pads at the same time, it is necessary to make the array of a plurality of pads formed on a circuit board (e.g. BGA board) accurately coincide with the array of a plurality of solder ball attracting nozzles set to a ball carrier. Recently, an apparatus is proposed which arrays a plurality of circuit boards and simultaneously mounts solder balls on a plurality of pads present on the circuit boards. In this case, it is requested that alignment of arrays is performed at higher accuracy.
That is, when arranging a plurality of circuit boards, it is necessary to make the positional relation between all pads included in the circuit boards accurately coincide with the positional relation between a plurality of solder ball attracting nozzles. FIG. 5 is a drawing to show the outline of a conventional BGA transferring apparatus used to perform the above alignment.
In FIG. 5, CSP housing portions 51 are formed like a grid on an exclusive tray 50 at almost equal intervals. Moreover, a plurality of circuit boards (e.g. CSPs) 52 of a semiconductor package according to a BGA structure before solder balls are mounted are arranged on the respective CSP housing portions 51.
An attracting nozzle 53 vacuum-attracts and picks up the CSPs 52 on the exclusive tray 50 one by one and successively transfers them to a carrying tray 55 in which an adhesive layer is formed on the upside of an aluminum plate. In this case, the picked-up CSP 52 is photographed by a camera 54 to recognize the outline of the CSP 52 through image processing. Moreover, based on the recognition result, a plurality of CSPs 52 are arranged on CSP mounting portions 56 formed on the carrying tray 55 at almost equal intervals so that all the CSPs 52 turn to the same direction.
Thus, the carrying tray 55 is transferred to the solder ball transferring apparatus while the CSPs 52 are arranged at equal intervals to transfer solder balls to pads on each CSP 52. By making the array of a plurality of solder ball attracting nozzles of the solder ball transferring apparatus same as the array of a plurality of pads present on each CSP 52 arranged at equal intervals, it is possible to simultaneously mount solder balls on a plurality of CSPs 52.
As described above, conventionally, when transferring a plurality of CSPs 52, the CSPs 52 are arranged at equal intervals by recognizing the package shape (outline) of the CSPs 52. However, not all CSPs 52 are present so that pads are arranged at the same position on their package without being unevenly distributed. Therefore, even if arranging a plurality of CSPs 52 through outline recognition, the pads included in the CSPs 52 are not always arranged at correct positions and therefore, there is a problem that the accuracy of alignment of pads is bad.
For example, as shown in FIG. 6, when arranging three CSPs 52 in which a plurality of pads 57 are not arranged at the same position in accordance with outline recognition, the CSPs 52 are accurately arranged at equal intervals but the important pads 57 are not accurately arranged as a whole and the arrangement of the pads 57 does not coincide with the arrangement of solder ball attracting nozzles. Therefore, a mounting error may occur that solder balls cannot be accurately mounted on each pad.